# Explain why enzymes can speed up reactions in living cells without being used up themselves.

> Edexcel GCSE Biology (1BI0) — 1.4 Enzymes · Explain · 4 marks

> Enzymes are biological catalysts found in all living organisms. A single enzyme molecule can be used repeatedly to catalyse the same reaction.

## Mark scheme (4 marks)

1. Enzymes are biological catalysts / they lower the activation energy needed for a reaction
2. The substrate binds to the active site of the enzyme to form an enzyme-substrate complex
3. After the reaction, the products are released from the active site
4. The enzyme is unchanged / the active site is unchanged after the reaction, so the same enzyme can catalyse the same reaction again

## Related

- [Revision notes for Edexcel GCSE Biology (1BI0)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

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